just moved in to a fifth floor apt.. doing some painting and a little work. noticed a very loud popping in the floor near a corner in the center of apt. there is a crack in the corner(in the plaster and paint) that goes up to the ceiling. and the baseboards are about a half inch off. i cant tell how old the crack is but it looks newish. this is the top floor of a old new york apt... should i be worried? thanks.